Method of manufacturing oxide superconducting wire

ABSTRACT

The inventive method of manufacturing an oxide superconducting wire comprises a step (S 1 , S 2 ) of preparing a wire formed by covering raw material powder of an oxide superconductor with a metal and a step (S 4 , S 6 ) of heat-treating the wire in a pressurized atmosphere, and the total pressure of the pressurized atmosphere is at least 1 MPa and less than 50 MPa. Thus, formation of voids between oxide superconducting crystals and blisters of the oxide superconducting wire is suppressed while the partial oxygen pressure can be readily controlled in the heat treatment, whereby the critical current density can be improved.

TECHNICAL FIELD

The present invention relates to a method of manufacturing an oxidesuperconducting wire, and more particularly, it relates to a method ofmanufacturing an oxide superconducting wire from a wire formed bycovering raw material powder of an oxide superconductor with a metal.

BACKGROUND TECHNIQUE

In general, a method of obtaining an oxide superconducting wire byheat-treating a wire formed by charging a metal tube with raw materialpowder of an oxide superconductor and thereafter wiredrawing and rollingthe metal tube for sintering the raw material powder of the oxidesuperconductor is known as a method of manufacturing an oxidesuperconducting wire. However, the wire is blistered in theaforementioned heat treatment step for sintering, to disadvantageouslyreduce the superconductivity of the obtained oxide superconducting wire.

Japanese Patent Laying-Open No. 5-101723 proposes a method ofmanufacturing an oxide superconducting wire by heat-treating a metaltube charged with powder of an oxide superconductor or a flattened bodythereof for sintering the powder of the oxide superconductor. Theaforementioned gazette describes that a wire having excellentsuperconductivity is obtained according to this method by performing apressure heat treatment.

More specifically, it is attempted to store a metal tube charged withpowder of an oxide superconductor in a heat-resistant/pressure-resistantclosed vessel for preventing blistering in sintering by increasing theinternal pressure following temperature rise in the closed vessel. Theaforementioned gazette describes that the current internal pressure canbe obtained from a state equation of gas or the like, and an internalpressure of about 4 atm. can be obtained with a heating temperature ofabout 900° C., for example.

Japanese Patent No. 2592846 (Japanese Patent Laying-Open No. 1-30114)proposes a method of manufacturing an oxide superconducting conductor byholding a metal tube charged with oxide superconducting powder or thelike in a high-pressure state at least either in a heat treatment orafter the heat treatment. The aforementioned gazette describes thatpartial separation on the interface between the oxide superconductor andthe metal tube caused in sintering can be prevented according to thismethod by setting the metal tube in the high-pressure state.

More specifically, the metal tube charged with the oxide superconductingpowder can be press-fitted to a sintered body by holding the metal tubein a high-pressure state of 500 to 2000 kg/cm² (about 50 to 200 MPa) atleast either in the heat treatment or after the heat treatment. Thus,when the superconductor partially causes quenching, heat resulting fromthis quenching can be quickly removed. In addition, it is also possibleto prevent deterioration of the superconductivity resulting from aseparation part forming a stress concentration part causing distortion.

In Japanese Patent Laying-Open No. 5-101723, however, the internalpressure obtained following temperature rise in the closed vessel isabout 4 atm (0.4 MPa). Thus, voids are formed between oxidesuperconducting crystals in sintering, to disadvantageously reduce thecritical current density.

Further, the oxide superconducting wire cannot be sufficiently inhibitedfrom blistering caused in sintering due to the internal pressure ofabout 4 atm (0.4 MPa), and hence the critical current density is alsodisadvantageously reduced.

In the method according to Japanese Patent No. 2592846, it is difficultto control the partial oxygen pressure in the heat treatment due toapplication of the excessively high pressure of 500 to 2000 kg/cm²(about 50 MPa to 200 MPa), to reduce the critical current density.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a method ofmanufacturing an oxide superconducting wire capable of improving thecritical current density by suppressing formation of voids between oxidesuperconducting crystals and blisters of the oxide superconducting wirewhile simplifying partial oxygen pressure control in a heat treatment.

The inventive method of manufacturing an oxide superconducting wire hasthe following characteristics:

A wire formed by covering raw material powder of an oxide superconductorwith a metal is prepared. The wire is heat-treated in a pressurizedatmosphere. The total pressure of the pressurized atmosphere is at least1 MPa and less than 50 MPa.

According to the inventive method of manufacturing an oxidesuperconducting wire, plastic flow and creep deformation are caused onsuperconducting crystals formed in a heat treatment due to the highpressure of at least 1 MPa outside the wire, thereby reducing the numberof voids between the oxide superconducting crystals. Further, gas inclearances of oxide superconducting crystal powder formed in the heattreatment or gas adhering to the oxide superconducting crystal powderformed in the heat treatment can be inhibited from expansion in the heattreatment due to the pressure from outside a metal tube, wherebyformation of blisters in the oxide superconducting wire is suppressed.Consequently, the critical current density is improved.

In order to form a stable oxide superconducting phase, it is necessaryto regularly control the partial oxygen pressure in a constant rangeregardless of the value of the total pressure in the pressurizedatmosphere. When the total pressure in the pressurized atmosphereexceeds 50 MPa in this case, however, the partial oxygen pressure withrespect to the total pressure is reduced. Thus, the value of the oxygenconcentration in the pressurized atmosphere is so remarkably reducedthat it is disadvantageously difficult to control the partial oxygenpressure due to strong influence by a measurement error or the like.According to the inventive method of manufacturing an oxidesuperconducting wire, the heat treatment is performed in the pressurizedatmosphere of less than 50 MPa so that the partial oxygen pressure withrespect to the total pressure in the pressurized atmosphere is notexcessively reduced but the value of the oxygen concentration in thepressurized atmosphere remains high to some extent, whereby control ofthe partial oxygen pressure is simplified substantially with noinfluence by a measurement error or the like.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the heat-treating step is carried out by hotisostatic pressing (HIP).

Thus, the oxide superconducting wire is so isotropically pressurizedthat the wire is homogeneously prevented from voids and blisters.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the oxide superconductor is a Bi—Pb—Sr—Ca—Cu—Ooxide superconductor including a Bi2223 phase containing bismuth, lead,strontium, calcium and copper in atomic ratios of (bismuth andlead):strontium:calcium:copper approximately expressed as 2:2:2:3.

Thus, formation of voids between crystals and blisters of the oxidesuperconducting wire is suppressed, whereby the critical current densitycan be improved.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the heat-treating step is carried out in an oxygenatmosphere, and the partial oxygen pressure is at least 0.003 Pa and notmore than 0.02 Pa.

Thus, the partial oxygen pressure is so kept in the range of at least0.003 MPa and not more than 0.02 MPa that a stable oxide superconductingphase is formed and the critical current density can be improved.Hetero-phases are formed if the partial oxygen pressure exceeds 0.02MPa, while the oxide superconducting phase is hardly formed and thecritical current density is reduced if the partial oxygen pressure isless than 0.003 MPa.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the partial oxygen pressure is controlled toincrease following temperature rise in the pressurized atmosphere at aheat-up time before the heat treatment in the heat-treating step.

The value of the partial oxygen pressure optimum for forming an oxidesuperconducting phase increases following the temperature rise. Thus,the partial oxygen pressure reaches a proper value also at the heat-uptime before the heat treatment in the heat-treating step, whereby astable oxide superconducting phase is so formed that the criticalcurrent density can be improved.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the total pressure in the pressurized atmosphereis controlled to be constant in the heat treatment.

In the heat treatment, the total pressure may exhibit a reductiontendency due to consumption of oxygen gas resulting from oxidation of abearer supporting the wire in a pressure vessel, hunting in pressurecontrol of a pressure regulator such as a dwell valve or pressurefluctuation in introduction of gas added for compensating for consumedoxygen. When abrupt decompression is thereby caused in the vessel, thepressure in the wire increases as compared with that outside the wire,to blister the wire. According to the preferred aspect of the presentinvention, however, the total pressure in the heat treatment iscontrolled to be constant, whereby the wire can be prevented fromformation of blisters resulting from abrupt decompression in the heattreatment.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the heat-treating step is carried out in an oxygenatmosphere, while the partial oxygen pressure in the heat treatment iscontrolled to be constant in a fluctuation range within 10%.

Thus, the partial oxygen pressure can be kept in the range of thepartial oxygen pressure optimum for forming an oxide superconductingphase, whereby a stable oxide superconducting phase is so formed thatthe critical current density can be improved.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, gas is injected to compensate for reduction of thepressure resulting from temperature reduction at a temperature reductiontime immediately after the heat treatment.

Pressure reduction follows temperature change at the temperaturereduction time immediately after the heat treatment. If a heating vesselis abruptly decompressed at this time, the pressure in the wireincreases as compared with that outside the wire, to blister the wire.According to the preferred aspect of the present invention, however, thegas is injected to compensate for pressure reduction resulting fromtemperature reduction, whereby the wire can be prevented from formationof blisters resulting from abrupt decompression at the temperaturereduction time immediately after the heat treatment.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the metal covering the raw material powderincludes silver, and the decompression speed at the temperaturereduction time immediately after the heat treatment is controlled to notmore than 0.05 MPa/min. when the ratio (hereinafter referred to as thesilver ratio) of the area of a metal portion to the area of an oxidesuperconductor portion in a cross section of the wire after theheat-treating step is 1.5.

Thus, the effect of preventing the wire from formation of blistersresulting from abrupt decompression is further remarkable when thesilver ratio is 1.5.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the decompression speed for the total pressure inthe pressurized atmosphere is controlled to not more than 0.05 MPa/min.if the temperature in the atmosphere is at least 200° C. when the metalcovering the raw material powder includes silver and the silver ratio is1.5.

If the heating vessel is abruptly decompressed when the temperature inthe atmosphere is at least 200° C., the pressure in the wire increasesas compared with that outside the wire, to blister the wire. Thus, theeffect of inhibiting the wire from formation of blisters resulting fromabrupt decompression in the heat-treating step (before the heattreatment, in the heat treatment and after the heat treatment) isfurther remarkable when the silver ratio is 1.5.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the decompression speed at the temperaturereduction time immediately after the heat treatment is controlled to notmore than 0.03 MPa/min. when the metal covering the raw material powderincludes silver and the silver ratio is 3.0.

Thus, the effect of preventing the wire from formation of blistersresulting from abrupt decompression is further remarkable when thesilver ratio is 3.0.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the decompression speed for the total pressure inthe pressurized atmosphere is controlled to not more than 0.03 MPa whenthe metal covering the raw material powder includes silver, the silverratio is 3.0 and the temperature in the atmosphere is at least 200° C.in the heat-treating step.

If the heating vessel is abruptly decompressed when the temperature inthe atmosphere is at least 200° C., the pressure in the wire increasesas compared with that outside the wire, to blister the wire. Thus, theeffect of inhibiting the wire from formation of blisters resulting fromabrupt decompression in the heat-treating step (before the heattreatment, in the heat treatment and after the heat treatment) isfurther remarkable when the silver ratio is 3.0.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the decompression speed for the total pressure inthe pressurized atmosphere is controlled to not more than 0.05 MPa whenthe total pressure in the pressurized atmosphere is at least 1 MPa inthe heat-treating step.

If the heating vessel is abruptly decompressed when the total pressurein the atmosphere is at least 1 MPa, the pressure in the wire increasesas compared with that outside the wire, to blister the wire. Thus, theeffect of inhibiting the wire from formation of blisters resulting fromabrupt decompression in the heat-treating step (before the heattreatment, in the heat treatment and after the heat treatment) isfurther remarkable.

Preferably, the aforementioned method of manufacturing an oxidesuperconducting wire further comprises a step of rolling the wire with aroll after the step of preparing the wire and before the heat-treatingstep, and the integumentary thickness of the wire after the rolling stepis at least 200 μm.

Pinholes are mainly formed to pass from outside up to oxidesuperconductor filaments since the surface of the wire is roughened dueto friction between the wire and the roll for the rolling. When the wireis so rolled that the integumentary thickness of the oxidesuperconducting wire is at least 200 μm in every portion after therolling step, however, no holes pass from outside up to the oxidesuperconductor filaments even if the surface of the wire is roughened byrolling, and hence no pinholes are formed. Thus, formation of voids andblisters is suppressed due to the aforementioned heat-treating step, sothat the critical current density can be improved. Throughout thespecification, the term “pinholes” denotes holes, having diameters of atleast 100 μm, passing from outside up to oxide superconducting wirefilaments. The term “wire having pinholes” denotes a wire of 4 mm by 10mm including at least two holes having diameters of at least 100 μm.

Preferably, the aforementioned method of manufacturing an oxidesuperconducting wire further comprises a step of bonding silver or asilver alloy to the surface of the said wire before the step ofpreparing the wire and after the heat-treating step.

In order to increase the quantity of a superconducting current feedableper unit area, the silver ratio of the oxide superconducting wire isminimized. However, the integumentary thickness of a wire having a smallsilver ratio cannot be increased due to a small ratio of a metalportion. Particularly in a wire having an integumentary thickness ofless than 200 μm after the heat-treating step, pinholes are readilyformed in treatment such as rolling before the heat-treating step. Inthe wire having pinholes, pressurizing gas penetrates into the wirethrough the pinholes also when the heat-treating step is carried out inthe aforementioned pressurized atmosphere. Therefore, no difference iscaused between the internal and external pressures of the wire, leadingto a small effect of preventing reduction of the critical currentdensity by suppressing formation of voids and blisters bypressurization. When silver or a silver alloy is bonded to the surfaceof the wire after the step of preparing the wire and before theheat-treating step, therefore, pinholes are covered with the silver orthe silver alloy to disappear from the surface. Therefore, theheat-treating step is carried out on the wire from which pinholes arepreviously removed, whereby the pressurizing gas does not penetrate intothe wire through pinholes in the heat-treating step. Thus, formation ofvoids and blisters is suppressed in the aforementioned heat-treatingstep in the pressurized atmosphere, whereby the critical current densitycan be improved.

Preferably, the aforementioned method of manufacturing an oxidesuperconducting wire further comprises a step of rolling the wire with aroll after the step of preparing the wire and before the heat-treatingstep, and surface roughness Ry of a portion of the roll coming intocontact with the wire is not more than 320 μm.

Thus, friction between the wire and the roll is so reduced that thesurface of the wire is hardly roughened and the wire is obtained with nopinholes regardless of the integumentary thickness of the wire.Therefore, the pressurizing gas does not penetrate into the wire throughpinholes in the heat-treating step. Thus, formation of voids andblisters is suppressed due to the aforementioned heat-treating step inthe pressurized atmosphere regardless of the integumentary thickness ofthe wire, so that the critical current density can be improved. The term“surface roughness Ry” denotes the maximum height of irregularitiesdefined in JIS (Japanese Industrial Standards).

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the pressure is controlled to increase stepwisefollowing temperature rise in the atmosphere at a heat-up time beforethe heat treatment in the heat-treating step.

When the wire has pinholes, the pressurizing gas penetrates into thewire through the pinholes also when the heat-treating step is carriedout in the pressurized atmosphere by ordinary pressurization, and henceno difference is caused between the internal and external pressures ofthe wire, leading to a small effect of preventing reduction of thecritical current density by suppressing formation of voids and blistersby pressurization. When the pressure is controlled to increase stepwisefollowing temperature rise in the atmosphere, however, the externalpressure increases before the pressurizing gas penetrates into the wirethrough the pinholes. Thus, pressure difference is caused between theinternal and external pressures of the wire so that formation of voidsand blisters is suppressed and the critical current density can beimproved regardless of whether or not the wire has pinholes before theheat-treating step.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the total pressure of the atmosphere is controlledto increase at a speed of at least 0.05 MPa/min. in temperature risebefore the heat treatment in the heat-treating step.

The inventors have found that the speed of pressurizing gas penetratinginto a wire through pinholes in a step of heat-treating the wire is lessthan about 0.05 MPa/min. When the total pressure of the atmosphere iscontrolled to continuously increase at the speed of at least 0.05MPa/min. in temperature rise before the heat treatment, therefore, it ispossible to regularly continuously keep the pressure in the atmospherehigher than the pressure in the wire. Thus, compressive force can beapplied to the wire in the temperature rise before the heat treatmentregardless of whether or not the wire has pinholes before theheat-treating step, whereby formation of voids and blisters issuppressed. Consequently, reduction of the critical current density canbe effectively suppressed due to the heat treatment in the pressurizedatmosphere of at least 1 MPa and less than 50 MPa.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the total pressure in the atmosphere is controlledto continuously increase in the heat treatment in the heat-treatingstep.

Thus, equalization of the pressure in the wire and that in theatmosphere can be retarded in the heat treatment, and it is possible tofurther continuously keep the state where the pressure in the atmosphereis higher than that in the wire. Therefore, formation of voids andblisters is suppressed in the heat treatment, and reduction of thecritical current density can be effectively suppressed due to the heattreatment in the pressurized atmosphere of at least 1 MPa and less than50 MPa.

Preferably, the aforementioned method of manufacturing an oxidesuperconducting wire further comprises a step of rolling the wire afterthe step of preparing the wire and before the heat-treating step, andthe draft of the wire in the rolling step is not more than 84%,preferably not more than 80%.

When the step of heat-treating the wire is carried out in thepressurized atmosphere of at least 1 MPa and less than 50 MPa, the oxidesuperconducting wire is compressed also in the heat-treating step. Alsowhen the step of rolling the wire is carried out with the draft of notmore than 84% lower than the conventional draft, therefore, the rawmaterial powder is compressed in the subsequent heat-treating step andhence the density of the superconducting filaments can be increased as aresult. On the other hand, the step of rolling the step is carried outwith the draft of not more than 84% lower than the conventional draft sothat voids are hardly formed in the raw material powder, wherebyformation of voids extending perpendicularly to the longitudinaldirection of the oxide superconducting wire can be suppressed. Thecritical current density of the oxide superconducting wire can beimproved for the aforementioned reasons. Further, the step of rollingthe wire is so carried out with the draft of not more than 80% that novoids are formed in the raw material powder, whereby formation of voidsextending perpendicularly to the longitudinal direction of the oxidesuperconducting wire can be further suppressed.

In this specification, the draft (%) is defined in the followingequation:Draft(%)=(1−thickness of rolled wire/thickness of unrolled wire) ×100

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, a plurality of heat treatments are performed onthe wire, and at least one heat treatment among the plurality of heattreatments is carried out in a pressurized atmosphere having a totalpressure of at least 1 MPa and less than 50 MPa.

Thus, it is possible to suppress formation of voids between oxidesuperconducting crystals and blisters of the oxide superconducting wirecaused in the heat treatment.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is a partially fragmented perspective view conceptually showingthe structure of an oxide superconducting wire.

FIG. 2 is a diagram showing a manufacturing step for an oxidesuperconducting wire.

FIG. 3 is a schematic sectional view of a hot isostatic pressing (HIP)apparatus.

FIGS. 4A to 4D are conceptual diagrams showing behavior of voids betweenoxide superconducting crystals stepwise.

FIG. 5 is a diagram showing the relation between the total pressure P(MPa) of a pressurized atmosphere and the number (/10 m) of blisters ofa wire.

FIG. 6 is a diagram showing total pressures and partial oxygen pressuresas to a gas mixture containing about 80% of nitrogen and about 20% ofoxygen.

FIG. 7 is a diagram showing the relation between total pressures andoxygen concentration values in a case of setting the partial oxygenpressure constant.

FIG. 8A is a diagram showing the relation between time and temperaturesof a wire in a case of controlling a decompression speed immediatelyafter a heat treatment, and FIG. 8B is a diagram showing the relationbetween time and total pressures in a vessel in the case of controllingthe decompression speed immediately after the heat treatment.

FIG. 9A is a graph showing thicknesses of wires of oxide superconductingwires having no pinholes before and after a heat treatment in apressurized atmosphere, and FIG. 9B is a graph showing thicknesses ofwires of oxide superconducting wires having pinholes.

FIG. 10 is a partially fragmented perspective view conceptually showingthe structure of an oxide superconducting wire having pinholes.

FIG. 11 is a schematic sectional view showing a rolling method in asecond embodiment.

FIG. 12 is a diagram showing another manufacturing step for an oxidesuperconducting wire.

FIG. 13 is a partially fragmented sectional view conceptually showingthe structure of an oxide superconducting wire after a step of platingthe wire with silver or a silver alloy.

FIG. 14 is a diagram showing the relation between temperatures andpressures in a heat treatment and time in a fourth method according tothe second embodiment.

FIG. 15A is a diagram showing the relation between temperatures in aheat-treating step and time in a case where a silver ratio in the secondembodiment is 1.5, FIG. 15B is a diagram showing the relation betweenpressures in the heat-treating step and time in the case where thesilver ratio in the second embodiment is 1.5, FIG. 15C is a diagramshowing the relation between oxygen concentrations in the heat-treatingstep and time in the case where the silver ratio in the secondembodiment is 1.5, and FIG. 15D is a diagram showing the relationbetween partial oxygen pressures in the heat-treating step and time inthe case where the silver ratio in the second embodiment is 1.5.

FIG. 16 is a diagram showing the relation between temperatures andpressures in a heat-treating step and time according to a fifth methodin the second embodiment.

FIG. 17 is a diagram showing the optimum combination of a temperatureand a partial oxygen pressure in a heat treatment.

FIG. 18 is a partially fragmented perspective view conceptually showingthe structure of an oxide superconducting wire having remaining voids.

FIG. 19 is a diagram schematically showing the relation between draftsand critical current densities in primary rolling in an oxidesuperconducting wire.

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are now described with reference tothe drawings.

First Embodiment

A multifilamentary oxide superconducting wire, for example, is describedwith reference to FIG. 1. An oxide superconducting wire 1 has aplurality of oxide superconductor filaments 2 extending in thelongitudinal direction and a sheath part 3 covering the same. Thematerial for each of the plurality of oxide superconductor filaments 2preferably has a Bi—Pb—Sr—Ca—Cu—O composition, for example, and amaterial including a Bi2223 phase having atomic ratios of (bismuth andlead):strontium:calcium:copper approximately expressed as 2:2:2:3 isoptimum in particular. The material for the sheath part 3 consists ofsilver, for example.

While a multifilamentary wire has been described in the above, an oxidesuperconducting wire having a single-core wire structure comprising asingle oxide superconductor filament 2 covered with a sheath part 3 mayalternatively be employed.

A method of manufacturing the aforementioned oxide superconducting wireis now described.

Referring to FIG. 2, raw material powder of an oxide superconductor isfirst charged into a metal tube (step S1). The raw material powder ofthe oxide superconductor consists of a material including a Bi2223phase, for example.

Silver or a silver alloy having high heat conductivity is preferablyemployed for the metal tube. Thus, heat generated when thesuperconductor partially causes quenching can be quickly removed fromthe metal tube.

Then, the metal tube charged with the raw material powder is worked intoa wire having a desired diameter by wiredrawing (step S2). Thus, a wireformed by covering the raw material powder of the oxide superconductorwith a metal is obtained. Primary rolling is performed on this wire(step S3), followed by a first heat treatment (S4). An oxidesuperconducting phase is generated from the raw material powder due tothese operations. Secondary rolling is performed on this heat-treatedwire (step S5). Thus, voids resulting from the first heat treatment areremoved. A second heat treatment is performed on the secondarily rolledwire (step S6). Sintering of the oxide superconducting phase progresseswhile the oxide superconducting phase is simultaneously converted to asingle phase through the second heat treatment.

The oxide superconducting wire shown in FIG. 1, for example, can bemanufactured according to the aforementioned manufacturing method.

In this embodiment, at least either the first heat treatment (step S4)or the second heat treatment (step S6) is performed in a pressurizedatmosphere to which a pressure of at least 1 MPa and less than 50 MPa isapplied as the total pressure.

The heat treatment in this pressurized atmosphere is performed by hotisostatic pressing (HIP), for example. This hot isostatic pressing isnow described.

Referring to FIG. 3, an apparatus 13 for performing hot isostaticpressing is constituted of a pressure vessel cylinder 6, a top cover 5and a bottom cover 11 closing both ends of the pressure vessel cylinder6, a gas inlet 4 provided on the top cover 5 for introducing gas intothe pressure vessel cylinder 6, a heater 9 heating a treated object 8, athermal barrier 7 and a bearer 10 supporting the treated object 8.

According to this embodiment, the bearer 10 supports the wire obtainedby charging the raw material powder into the metal tube and thereafterwiredrawing/rolling the same as the treated object 8 in the pressurevessel cylinder 6. In this state, prescribed gas is introduced into thepressure vessel cylinder 6 through the gas inlet 4 thereby forming apressurized atmosphere of at least 1 MPa and less than 50 MPa in thepressure vessel cylinder 6 and heating the wire 8 with the heater 9 to aprescribed temperature under this pressurized atmosphere. This heattreatment is preferably performed in an oxygen atmosphere, and thepartial oxygen pressure is preferably at least 0.003 MPa and not morethan 0.02 MPa. Thus, the wire 8 is subjected to the heat treatment byhot isostatic pressing.

According to this embodiment, the heat treatment is performed in thepressurized atmosphere of at least 1 MPa and less than 50 MPa ashereinabove described, to mainly attain the following three effects:

First, the number of voids formed between oxide superconducting crystalsin the heat treatment can be reduced.

The inventors have found that the number of voids formed between oxidesuperconducting crystals mainly in a heat treatment can be remarkablyreduced by performing the heat treatment in a pressurized atmosphere ofat least 1 MPa as compared with a case of less than 1 MPa.

Referring to FIGS. 4A to 4D, the contact area between oxidesuperconducting crystals formed in a heat treatment increases due toplastic flow when the heat treatment is performed in a pressurizedatmosphere, to reduce the number of voids of several μm to several 10 μmpresent between the superconducting crystals (FIG. 4A→FIG. 4B). Whenheld in this state, creep deformation is caused as shown in FIG. 4C sothat voids present on the junction interface shrink while acontaminative portion such as an oxide film is partiallybroken/decomposed to cause diffusion of atoms and progress sintering.The voids between the superconducting crystals finally substantiallydisappear as shown in FIG. 4D, and a stable junction interface isformed.

To feed a current to a superconducting wire is to feed a current betweensuperconducting crystals constituting the superconducting wire. Whatlimits the quantity of a current feedable while keeping asuperconducting state (causing no electric resistance) in a coolant(e.g., liquid nitrogen or helium, or a refrigerator) for employing asuperconducting wire is generally the junction between superconductingcrystals having a weak superconducting state (the superconductingcrystals have stronger superconductivity than the junction between thecrystals). Voids inevitably remain in the junction between thesuperconducting crystals in ordinary atmospheric baking. When the numberof voids between the superconducting crystals is reduced, therefore, theperformance of the superconducting wire is so improved that reduction ofthe critical current density can be prevented.

More specifically, the sintering density of an oxide superconductorheat-treated in the atmospheric pressure was 80 to 90% as to an oxidesuperconducting wire containing a Bi2223 phase, while the sinteringdensity of an oxide superconductor prepared according to the inventivemanufacturing method by setting the total pressure of a pressurizedatmosphere to 10 MPa was 93 to 96%, and reduction of the number of voidsformed between oxide superconducting crystals was observed.

Second, the oxide superconducting wire can be prevented from blistersformed in the heat treatment.

The inventors have investigated the number of blisters formed in aheat-treated wire when varying the total pressure for heat-treating anoxide superconducting wire in a pressurized atmosphere. Referring toFIG. 5, it is understood that the number of blisters in the oxidesuperconducting wire is remarkably reduced when the total pressure ofthe pressurized atmosphere exceeds 0.5 MPa and the blisters in the oxidesuperconducting wire substantially disappear when the total pressure isin excess of 1 MPa. Such results have been obtained conceivably for thefollowing reason:

Powder of an oxide superconductor is generally charged into a metal tubeat a filling factor of about 80% of the theoretical density beforesintering, and hence gas is present in voids of the powder. The gas inthe voids of the powder causes cubic expansion when reaching a hightemperature in a heat treatment, to blister the wire. According to thisembodiment, however, the heat treatment is performed in the pressurizedatmosphere of at least 1 MPa, and hence the pressure outside the metaltube can increase beyond that in the metal tube. Thus, the wire isconceivably prevented from blisters caused by the gas in the voids ofthe powder.

The inventors have further studied the cause for blisters of the wire,to also recognize that adsorbates such as carbon (C), water (H₂O) andoxygen (O₂) adhering to the raw material powder of the oxidesuperconductor are vaporized during sintering and the volume in themetal tube expands due to this gas to blister the wire. However, suchblisters the wire resulting from vaporization of the adsorbates to thepowder can also conceivably be prevented since the external pressure canincrease beyond the intermetallic internal pressure by performing theheat treatment in the pressurized atmosphere of at least 1 MPa.

Thus, it is conceivably possible to substantially remove not onlyblisters resulting from the gas present in the voids of the raw materialpowder of the oxide superconductor but also blisters resulting fromvaporization of the adsorbates adhering to the surfaces of particlesthereof. The blisters of the oxide superconducting wire cause reductionof the critical current density, and hence reduction of the criticalcurrent density can be prevented by preventing the wire from blisters.

Third, the partial oxygen pressure can be readily controlled in the heattreatment.

The inventors have found that a 2223 phase of a Bi-based oxidesuperconductor is stably formed when the partial oxygen pressure iscontrolled to at least 0.003 MPa and not more than 0.02 MPa regardlessof the total pressure. A hetero phase such as Ca₂PbO₄ is formed if thepartial oxygen pressure exceeds 0.02 MPa, while the Bi2223 phase ishardly formed and the critical current density is reduced if the partialoxygen pressure is less than 0.003 MPa.

Referring to FIG. 6, a Bi2223 phase is stably formed without controllingthe partial oxygen pressure when the total pressure of the pressurizedatmosphere is the atmospheric pressure of 1 atm (0.1 MPa), for example,since the partial oxygen pressure is equivalent to the level of 0.2 atm(0.02 MPa) shown by a dotted line. As the total pressure of thepressurized atmosphere increases to 2 atm, 3 atm . . . , however, thepartial oxygen pressure also increases to exceed the level of 0.2 atmshown by the dotted line. Consequently, the Bi2223 phase is not stablyformed. Therefore, the partial oxygen pressure must be controlled to atleast 0.003 MPa and not more than 0.02 MPa by varying the mixing ratioof oxygen gas in a gas mixture, as shown in FIG. 7. A dotted line inFIG. 7 shows the level of 0.2 atm (0.02 MPa) similarly to the dottedline in FIG. 6.

In practice, the partial oxygen pressure is controlled by monitoring thetotal pressure and the oxygen concentration. In other words, the partialoxygen pressure is calculated by multiplying the value of the totalpressure by the oxygen concentration.

Therefore, if the total pressure is 50 MPa, for example, the oxygenconcentration is 0.01% when the heat treatment is performed with apartial oxygen pressure of 0.005 MPa. Therefore, the injected gasmixture must be controlled by measuring the oxygen concentration of0.01%. However, the oxygen concentration of 0.01% is substantiallyidentical to a measurement error, and hence it is difficult to controlthe oxygen gas in the injected gas mixture by correctly measuring thisoxygen concentration. According to this embodiment, the total pressurein the pressurized atmosphere is set to less than 50 MPa so that theconcentration of the oxygen gas in the injected gas mixture can be kepthigh to some extent by reducing influence by a measurement error of theoxygen concentration, whereby the partial oxygen pressure can be readilycontrolled.

When performing the heat treatment in the pressurized atmosphere of atleast 1 MPa, the decompression speed is preferably controlled so that noabrupt decompression takes place in the pressurized atmosphere in theheat treatment and after the heat treatment.

When the heat treatment is performed in the pressurized atmosphere of atleast 1 MPa, external gas conceivably enters the wire through fine holeson the surface of the wire to provide the same pressure as the externalone. The inventors have found that emission of gas from the inner partcannot follow reduction of the external pressure and the internalpressure increases to form blisters when the external pressure isreduced due to abrupt decompression in such a high-pressure atmosphere.

In order to prevent such blisters, therefore, a gas mixture of inert gassuch as Ar (argon) or N₂ (nitrogen) and O₂ gas is preferably injectedinto the vessel in the heat treatment for setting the total pressureconstant. In temperature reduction immediately after the heat treatment,further, the gas mixture of the inert gas and the oxygen gas is injectedinto the vessel to compensate for reduction of the pressure resultingfrom temperature reduction. Formation of blisters resulting from abruptdecompression can be prevented by controlling the decompression speed inthe heat treatment and at the temperature reduction time immediatelyafter the heat treatment.

Referring to FIGS. 8A and 8B, the total pressure is controlled to beconstant as shown in FIG. 8B in a heat treatment (at a temperature ofabout 800° C.) shown in FIG. 8A. In other words, oxygen gas in a heatingvessel is consumed in the heat treatment due to oxidation of a bearersupporting a wire in the vessel, and hence the pressure in the vessel isreduced. In order to prevent this, a gas mixture is injected into thevessel for keeping the pressure constant. In temperature reduction (in atemperature range of about 800 to about 300° C.) immediately after theheat treatment shown in FIG. 8A, the gas mixture is injected into thevessel to compensate for reduction of the pressure resulting from thetemperature reduction as shown in FIG. 8B, for controlling thedecompression speed to not more than a constant level. In other words,the pressure of the gas also starts to abruptly lower on the basis ofthe state equation of gas due to abrupt reduction of the temperature,and hence temperature reduction must be loosened by injecting the gasmixture. When not more than 300° C., the temperature is low as comparedwith the case of about 800° C. to about 300° C., and hence the pressurein the wire is already sufficiently low. Therefore, the wire isconceivably not blistered also when the decompression speed is notcontrolled.

The inventors have found that the range of the decompression speednecessary for preventing the oxide superconducting wire from blistersvaries with the ratio (silver ratio) of the area of a metal portion tothe area of an oxide superconductor portion in a cross section of theheat-treated wire. In other words, the decompression speed intemperature reduction (in the range of 800° C. to 300° C.) immediatelyafter the heat treatment is preferably not more than 0.05 MPa/min. whenthe silver ratio is 1.5, and the decompression speed in temperaturereduction (in the temperature range of 800° C. to 300° C.) immediatelyafter the heat treatment is preferably not more than 0.03 MPa/min. whenthe silver ratio is 3.0.

Second Embodiment

Heat treatment conditions in FIGS. 9A and 9B are a total pressure of 20MPa, a partial oxygen pressure of 0.008 MPa, a temperature of 825° C. inan atmosphere, and a heat treatment time of 50 hours. Referring to FIG.9A, the thickness of an oxide superconducting wire having no pinholes isreduced by about 0.006 mm to 0.01 mm after a heat treatment. This isbecause formation of voids between oxide superconducting crystals andblisters of the oxide superconducting wire is suppressed due to the heattreatment in a pressurized atmosphere having the total pressure of 20MPa. Referring to FIG. 9B, on the other hand, the thickness of an oxidesuperconducting wire having pinholes is reduced only by about 0.002 mmto 0.005 mm after the heat treatment, and formation of voids betweenoxide superconducting crystals and blisters of the oxide superconductingwire is not sufficiently suppressed. The thickness of a portion (portionA) having pinholes in the wire increases after the heat treatment ascompared with that before the heat treatment.

Thus, it has been recognized that formation of voids and blisters can beeffectively suppressed by performing the heat treatment in the pressurerange (at least 1 MPa and less than 50 MPa) according to the firstembodiment when there are no pinholes while formation of voids andblisters cannot be sufficiently suppressed by merely performing the heattreatment in the pressure range (at least 1 MPa and less than 50 MPa)according to the first embodiment when there are pinholes.

In the heat treatment in the pressurized atmosphere according to thepresent invention, plastic flow and creep deformation take place in thesuperconducting crystals formed in the heat treatment due to the highpressure of at least 1 MPa outside the wire, whereby voids between theoxide superconducting crystals formed in the heat treatment aresuppressed. Further, the gas in the voids of the oxide superconductingcrystal powder formed in the heat treatment or the gas adhering to theoxide superconducting crystal powder formed in the heat treatment can beinhibited from expansion in the heat treatment due to the pressure fromoutside the metal tube, whereby the oxide superconducting wire isinhibited from formation of blisters. Consequently, reduction of thecritical current density resulting from voids and blisters is prevented.

In a wire having pinholes, however, pressurizing gas penetrates into thewire through the pinholes also when the aforementioned heat treatment inthe pressurized atmosphere is performed, and hence no difference iscaused between the internal and external pressures of the wire andformation of voids and blisters is not sufficiently suppressed bypressurization. Consequently, the effect of preventing reduction of thecritical current density is reduced.

In this regard, the inventors have made deep studies, to find techniquescapable of sufficiently suppressing formation of voids and blisters byproviding a wire having no pinholes before a heat treatment.

A first technique is to set the integumentary thickness of the oxidesuperconducting wire to at least 200 μm after the rolling (step S3 orS5) and before the heat treatment (step S4 or step S6) in FIG. 2.

A second technique is to set the surface roughness Ry of a portion,coming into contact with the wire, of the roll employed for the rolling(step S3 or S5) in FIG. 2 to not more than 320 μm.

The third technique is to plate the oxide superconducting wire withsilver or a silver alloy after the rolling (step S3 or S5) and beforethe heat treatment (step S4 or S6) in FIG. 2.

These techniques are now specifically described.

The inventors have found that no pinholes are formed in rolling (step S3or S5) when the integumentary thickness W of the oxide superconductingwire after the rolling (step S3 or S5) and before the heat treatment(step S4 or S6) in FIG. 2 is set to at least 200 μm in every portion asthe first technique. The term “integumentary thickness W” denotes thedistance W between each of oxide superconductor filaments 2 arrangedalong the outer peripheral portion in a section of a wire 1 and theouter surface of the wire 1 after rolling, as shown in FIG. 10. Thereason why no pinholes 14 are formed when the integumentary thickness Wis set to at least 200 μm is conceivably as follows:

The pinholes 14 are mainly formed to pass from outside up to the oxidesuperconductor filaments 2 since the surface of the wire 1 is rougheneddue to friction between the wire 1 and a roll for rolling. When theoxide superconducting wire 1 is so rolled that the integumentarythickness W thereof is at least 200 μm in every portion after therolling, however, no holes pass from outside up to the oxidesuperconductor filaments 2 even if the surface of the wire 1 isroughened due to the rolling, and hence no pinholes 14 are conceivablyformed. The remaining structure of FIG. 10 other than the above issubstantially identical to the structure shown in FIG. 1, and henceidentical members are denoted by identical reference numerals andredundant description is not repeated.

The inventors have found that, even if the integumentary thickness W ofthe rolled oxide superconducting wire is less than 200 μm, a wire havingno pinholes 14 is obtained before the heat treatment when theaforementioned second and third techniques are employed so thatformation of voids and blisters is consequently suppressed by the heattreatment in the pressurized atmosphere and reduction of the criticalcurrent density is effectively prevented.

Referring to FIG. 11, rolling is a working method passing a plate-shapedor bar-shaped material between a plurality of (generally two) rotatingrolls 15 for reducing the thickness or the sectional area thereof whilemolding the section to a target shape at the same time. In this rolling,an oxide superconducting wire 1 is drawn into the space between theplurality of rolls 15 due to frictional force from the rolls 15 anddeformed due to compressive force received from the surfaces 15 a of therolls 15.

In the second technique, the rolls 15 having surface roughness Ry of notmore than 320 μm on the surfaces 15 a coming into contact with the wire1 are employed in at least either the primary rolling (step S3) or thesecondary rolling (step S5) shown in FIG. 2.

When the surface roughness Ry of the surfaces 15 a of the rolls 15employed in the rolling is not more than 320 μm, friction between thewire 1 and the surfaces 15 a of the rolls 15 is so reduced that thesurface of the wire 1 is hardly roughened and the wire 1 is obtainedwith no pinholes regardless of the integumentary thickness of the wire1. Therefore, no pressurizing gas penetrates into the wire 1 throughpinholes in the heat-treating step. Thus, formation of voids andblisters is suppressed due to the aforementioned step of performing theheat treatment in the pressurized atmosphere regardless of theintegumentary thickness W of the wire 1, whereby reduction of thecritical current density is effectively prevented.

In the third technique, a step (step S11 or S12) of plating the surfaceof the wire with silver or a silver alloy is carried outer after rolling(step S3 or S5) and before a heat treatment (step S4 or S6), as shown inFIG. 12. The steps are substantially identical to those of the methodshown in FIG. 2 except that the plating step (step S11 or S12) is added,and hence corresponding steps are denoted by corresponding referencenumerals and redundant description is not repeated.

Referring to FIG. 13, the outer periphery of a sheath part 3 is platedwith silver or a silver alloy 16, so that externally opening pinholes 14are blocked with the silver or silver alloy 16. The remaining structureis substantially identical to the structure shown in FIG. 1, and henceidentical members are denoted by identical reference numerals andredundant description is not repeated.

In general, the silver ratio of the oxide superconducting wire 1 isminimized in order to increase the quantity of a superconducting currentfeedable per unit area. In the wire 1 having a small silver ratio,however, the integumentary thickness W cannot be increased due to thesmall ratio of a metal portion. Therefore, the integumentary thickness Wis less than 200 μm in the wire 1 having a small silver ratio, andpinholes 14 are readily formed in a treatment (e.g., rolling) before theheat-treating step. When the wire 1 has pinholes 14, formation of voidsand blisters is not sufficiently suppressed by pressurization asdescribed above. Consequently, the effect of preventing reduction of thecritical current density is reduced. Thus, the surface of the wire 1 isplated with the silver or silver alloy 16 before the heat-treating step,so that the pinholes 14 are blocked with the silver or silver alloy 16and disappear from the surface. Therefore, the heat-treating step iscarried out after the pinholes 14 disappear from the wire 1, whereby nopressurizing gas penetrates into the wire 1 through the pinholes 14 inthe heat-treating step. Thus, formation of voids and blisters issuppressed due to the aforementioned step of performing the heattreatment in the pressurized atmosphere regardless of the value of theintegumentary thickness W of the wire 1 and the value of the surfaceroughness Ry of the rolls 15 employed for rolling, for effectivelypreventing reduction of the critical current density.

The inventors have found that formation of voids and blisters issuppressed and reduction of the critical current density is effectivelyprevented also in the wire 1 having the pinholes 14 when a fourthtechnique or a fifth technique described below is employed. In thefourth technique, the pressure is controlled to increase stepwisefollowing temperature rise at a heat-up time before the heat treatmentin at least either the first heat treatment (step S4) or the second heattreatment (step S6) shown in FIG. 2. In the fifth technique, the totalpressure of the atmosphere is controlled to increase at a rate of atleast 0.05 MPa/min. at the heat-up time before the heat treatment in atleast either the first heat treatment (step S4) or the second heattreatment (step S6) shown in FIG. 2. In the heat treatment, the totalpressure in the atmosphere is controlled to continuously increase. Intemperature reduction immediately after the heat treatment, further,control is made to compensate for reduction of the pressure resultingfrom the temperature reduction (to add a pressure). First, the fourthtechnique is described.

Referring to FIG. 14, a heat treatment is performed under conditions ofa heat treatment temperature of 800° C. and a pressure of 20 MPa. Atthis time, the pressure is controlled to increase stepwise followingtemperature rise. In other words, the pressure is controlled to repeat aprocess of holding a prescribed pressure for a constant time, thereafterincreasing the pressure and holding the increased pressure for aconstant time again in pressure increase. More specifically, thepressure is held at about 7 MPa, about 10 MPa, about 12.5 MPa, about 15MPa and about 17 MPa for constant times in the pressure increaseprocess. The timing for increasing the pressure after holding the samefor a constant time is controlled on the basis of a measured value ofthe temperature in the atmosphere. In other words, the pressure iscontrolled by increasing the pressure to about 7 MPa at the roomtemperature, increasing the pressure to about 10 MPa when thetemperature reaches about 400° C., increasing the pressure to 12.5 MPawhen the temperature reaches 500° C., increasing the pressure to about15 MPa when the temperature reaches 600° C. and increasing the pressureto about 17 MPa when the temperature reaches 700° C. In order to form astable oxide superconducting phase, the partial oxygen pressure iscontrolled to be regularly in the range of 0.003 to 0.008 MPa.

In a wire having pinholes, pressurizing gas penetrates into the wirethrough the pinholes also when a step of performing a heat treatment ina pressurized atmosphere is carried out by ordinary pressing, and hencepressure difference between the internal and external pressures of thewire disappears to result in a small effect of preventing reduction ofthe critical current density resulting from voids and blisters bypressing. When the pressure is controlled to increase stepwise followingtemperature rise as in the fourth technique, however, the externalpressure increases before the pressing gas penetrates into the wirethrough pinholes. Thus, pressure difference is caused between theinternal and external pressures of the wire so that formation of voidsand blisters is suppressed and reduction of the critical current densityis effectively prevented regardless of whether or not the wire haspinholes before the heat-treating step.

Formation of voids and blisters in the wire can be further effectivelysuppressed by combining the following technique with the aforementionedfirst to fourth techniques. This technique is now described.

In this technique, the decompression speed of the total pressure in thepressurized atmosphere is controlled to be less than a constant speed inat least either the first heat treatment (step S4) or the second heattreatment (step S6) shown in FIG. 2 if the temperature in the atmosphereis at least 200° C. in the heat-treating step.

Referring to FIGS. 15A to 15D, the pressure is controlled to increasestepwise following temperature rise in the atmosphere at a heat-up timebefore the heat treatment, similarly to the aforementioned fourthtechnique. While it seems that no prescribed pressure is held for aconstant time in FIG. 15B, a pressure holding part merely seems to beomitted since the scale of an elapsed time in FIG. 15B is excessivelylarger than that in FIG. 14, and the prescribed pressure is held for aconstant time in practice similarly to the case of FIG. 14. Thetemperature and the pressure are set to 815° C. and 20 MPa respectivelydue to this heat-up step, and a heat treatment of 50 hours is performedin this state. At the heat-up time before the heat treatment and in theheat treatment, the decompression speed for the total pressure in thepressurized atmosphere is controlled to not more than 0.05 MPa/min. whenthe temperature in the atmosphere is at least 200° C. After the heattreatment, the temperature is reduced at a speed of 50° C./h. Also afterthe heat treatment, the decompression speed for the total pressure inthe pressurized atmosphere is controlled to not more than 0.05 MPa/min.when the temperature in the atmosphere is at least 200° C. If thetemperature reduction speed after the heat treatment is 50° C./h, thenatural decompression speed following temperature reduction is regularlynot more than 0.05 MPa/min., and hence the decompression speed may notbe controlled. Further, the oxygen concentration is kept at 0.04% beforethe heat treatment, in the heat treatment and after the heat treatment.Thus, the partial oxygen pressure is regularly in the range of 0.003 to0.008 MPa, so that a stable oxide superconducting phase is formable.

If the heating vessel is abruptly decompressed when the temperature inthe atmosphere is at least 200° C., the pressure in the wire increasesas compared with that outside the wire, to blister the wire. Therefore,the decompression speed for the total pressure in the pressurizedatmosphere is controlled to be less than a constant speed, so that theeffect of inhibiting the wire from formation of blisters resulting fromabrupt decompression during the heat treatment (before the heattreatment, in the heat treatment and after the heat treatment) isfurther remarkable.

As to a wire having a silver ratio of 3.0, the decompression speed iscontrolled to not more than 0.03 MPa/min. when the temperature in theatmosphere is a least 200° C.

The fifth technique is now described. In the fifth technique, the totalpressure of the atmosphere is controlled to continuously increase at aspeed of at least 0.05 MPa/min. at the heat-up time before the heattreatment in either the first heat treatment (step S4) or the secondheat treatment (step S6). In the heat treatment, the total pressure inthe atmosphere is controlled to continuously increase. In temperaturereduction immediately after the heat treatment, further, control is madeto compensate for reduction of the pressure resulting from temperaturereduction (to add a pressure).

Referring to FIG. 16, the pressure is loosely increased according to thestate equation of gas at the heat-up time before the heat treatment ifthe temperature of the atmosphere is not more than 700° C., for example.When the temperature of the atmosphere substantially exceeds 700° C.,the pressure in the atmosphere is increased to about 10 MPa. At thistime, the pressure in the atmosphere is increased at a blast with apressing speed of at least 0.05 MPa/min.

The inventors have found that the speed of pressurizing gas penetratinginto a wire through pinholes is less than about 0.05 MPa/min. when anoxide superconducting wire having the pinholes is heat-treated in apressurized atmosphere. Therefore, the pressure in the atmosphere can bekept higher than that in the wire at the heat-up time before the heattreatment by controlling the total pressure of the atmosphere tocontinuously increase at a speed of at least 0.05 MPa/min. at theheat-up time before the heat treatment.

Thereafter the temperature is kept at 830° C., for example, in the heattreatment. On the other hand, the pressure in the atmosphere iscontinuously increased. While the pressing speed in the heat treatmentis preferably as high as possible, the total pressure exceeds 50 MPa ifthe pressing speed is excessively high and hence the pressure must becontinuously increased at such a proper pressing speed that the totalpressure in the heat treatment does not exceed 50 MPa. Referring to FIG.16, the pressure is increased to about 30 MPa. Therefore, the time whenthe pressure in the wire and that in the atmosphere are equalized witheach other can be retarded from a time t1 to a time t2 as compared witha case where the pressure is kept constant in the heat treatment. Thus,the state where the pressure in the atmosphere is higher than that inthe wire can be continuously kept longer in the heat treatment.

Thereafter in temperature reduction immediately after the heattreatment, the pressure starts to lower according to the state equationof gas following reduction of the temperature in the atmosphere. At thistime, the pressure is controlled to compensate for reduction of thepressure resulting from temperature reduction (to add a pressure). Inorder to form a stable oxide superconducting phase, the partial oxygenpressure is controlled to be regularly in the range of 0.003 to 0.02MPa.

According to the fifth technique, the pressure in the atmosphereincreases beyond that in the wire at the heat-up time before the heattreatment, whereby compressive force can be applied to the wire.Further, the state where the pressure in the atmosphere is higher thanthat in the wire can be continuously kept longer in the heat treatment.Consequently, formation of voids and blisters is suppressed at theheat-up time before the heat treatment and in the heat treatment,whereby reduction of the critical current density can be effectivelysuppressed due to the heat treatment in the pressurized atmosphere of atleast 1 MPa and less than 50 MPa.

Third Embodiment

In order to further improve the critical current density of the oxidesuperconducting wire, the inventors have made deep studies as to theoptimum partial oxygen pressure at the heat-up time before the heattreatment and in the heat treatment. Thus, results shown in FIG. 17 havebeen obtained.

Referring to FIG. 17, it is understood that a stable oxidesuperconducting phase is formed and the critical current density isimproved in a temperature range of at least 815° C. and not more than825° C. if the partial oxygen pressure is 0.007 MPa, for example.Further, a stable oxide superconducting phase is formed and the criticalcurrent density is improved in a temperature range of at least 750° C.and not more than 800° C., preferably in a temperature range of at leastat least 770° C. and no more than 800° C. if the partial oxygen pressureis 0.0003 MPa, although this is not shown in the figure. In addition, astable oxide superconducting phase is formed and the critical currentdensity is improved in a set in the optimum partial oxygen pressurerange regardless of temperature fluctuation, whereby a stable oxidesuperconducting phase is formed so that the critical current density canbe improved.

From the above relation between the temperature and the partial oxygenpressure, the value of the partial oxygen pressure optimum for formingan oxide superconducting phase increases following temperature rise. Atthe heat-up time before the heat treatment, therefore, the partialoxygen pressure can be set in the range optimum for forming an oxidesuperconducting phase by controlling the partial oxygen pressure toincrease following temperature rise in the atmosphere. Thus, a stableoxide superconducting phase is formed so that the critical currentdensity can be improved

When a wire is held at a constant temperature in the heat treatment,fluctuation (error) of several ° C. is frequently caused in thetemperature. Considering the relation between this fluctuation of thetemperature and the optimum range of the partial oxygen pressure, theoptimum partial oxygen pressure is at least 0.006 MPa and not more than0.01 MPa when the wire is held at 822.5° C., for example, while theoptimum partial oxygen pressure is at least 0.007 MPa and not more than0.011 MPa when the temperature fluctuates to 825° C. When thetemperature fluctuates to 820° C., the optimum partial oxygen pressureis at least 0.005 MPa and not more than 0.009 MPa. In order to regularlyattain the optimum partial oxygen pressure despite such temperaturefluctuation, therefore, it follows that the partial oxygen pressure maybe controlled to be constant in a fluctuation range (slant line portionin FIG. 17) of at least 0.007 MPa and not more than 0.009 MPa when thewire is held at 822.5° C.

This fluctuation range of the partial oxygen pressure is about 10% ofthe value of the partial oxygen pressure. When the partial oxygenpressure in the heat treatment is controlled to be constant in thefluctuation range within 10%, therefore, the partial oxygen pressure canbe set in the optimum partial oxygen pressure range regardless oftemperature fluctuation, whereby a stable oxide superconducting phase isformed so that the critical current density can be improved.

Fourth Embodiment

In order to further improve the critical current density of the oxidesuperconducting wire, the inventors have controlled the decompressionspeed for the total pressure in the heat treatment to 0.05 MPa/min. andmade deep studies as to the relation between the value of the totalpressure and formation of blisters in the wire.

Raw material powder containing Bi, Pb, Sr, Ca and Cu in compositionratios of 1.82:0.33:1.92:2.01:3.02 was prepared. This raw materialpowder was heat-treated at 750° C. for 10 hours, and thereafterheat-treated at 800° C. for 8 hours. Thereafter powder obtained bypulverization was heat-treated at 850° C. for 4 hours, and thereafterpulverized again. Powder obtained by the pulverization was heat-treatedunder decompression, and thereafter charged into a metal tube of silverhaving an outer diameter of 36 mm and an inner diameter of 31 mm. Then,the metal tube charged with the powder was wiredrawn. Further, 61wiredrawn wires were bundled and engaged into a metal tube of 36 mm inouter diameter and 31 mm in inner diameter. Then, wiredrawing andprimary rolling were performed for obtaining a tapelike superconductingwire having a Bi2223 phase with a thickness of 0.25 mm and a width of3.6 mm. Then, a first heat treatment was performed on this wire. Thefirst heat treatment was performed in the atmosphere with a heattreatment temperature set to 842° C. and a heat treatment time set to 50hours. Then, secondary rolling was performed, followed by a second heattreatment. The second heat treatment was performed by setting thepartial oxygen pressure to 0.008 MPa, setting the heat treatmenttemperature to 825° C., setting the heat treatment time to 50 hours,controlling the decompression speed for the total pressure in the heattreatment to 0.05 MPa/min. and varying the total pressure as shown inTable 1. After the second heat treatment, presence/absence of blistersin the wire was investigated. Table 1 also shows the total pressure andpresence/absence of blisters in the wire.

TABLE 1 Total Pressure (MPa) Expansion of Wire 0.1 no 0.2 no 0.3 no 0.4no 0.5 no 0.8 no 1.0 yes 2.0 yes 3.0 yes 5.0 yes 10.0 yes 20.0 yes 30.0yes

From the results shown in Table l, the wire is blistered when the totalpressure is at least 1 MPa. Thus, it is necessary to control thedecompression speed in the pressurize atmosphere to not more than 0.05MPa/min. when the total pressure is at least 1 MPa, in order to inhibitthe wire from blisters.

Then, the heat treatment temperature for the second heat treatment wasset to 500° C., for similarly investigating presence/absence of blistersin the wire. Table 2 also shows the total pressure and presence/absenceof blisters of the wire.

TABLE 2 Total Pressure (MPa) Expansion of Wire 0.1 no 0.2 no 0.3 no 0.4no 0.5 no 0.8 no 1.0 yes 2.0 yes 3.0 yes 5.0 yes 10.0 yes 20.0 yes 30.0yes

From the results shown in Table 2, the wire is blistered if the totalpressure is at least 1 MPa also when the heat treatment temperature is500° C. Thus, it is necessary to control the decompression speed in thepressurize atmosphere to not more than 0.05 MPa/min. if the totalpressure is at least 1 MPa, in order to inhibit the wire from blistersalso when the heat treatment temperature is 500° C.

Fifth Embodiment

Referring to FIG. 18, voids large in the longitudinal direction(transverse direction in FIG. 18) substantially disappear while voids 20extending perpendicularly to the longitudinal direction slightly remainin a superconducting filament 2 of an oxide superconducting wire 1subjected to a heat treatment in a pressurized atmosphere having a totalpressure of at least 1 MPa and less than 50 MPa. FIG. 18 shows asingle-core oxide superconducting wire having a single superconductingfilament.

In other words, the inventors have found that the number of the voids 20extending perpendicularly to the longitudinal direction of the oxidesuperconducting wire 1 is hardly reduced by a heat treatment in apressurized atmosphere either. This is conceivably for the followingreason: In the pressurized atmosphere, a pressure is equally applied toall surfaces of the oxide superconducting wire. Oxide superconductingcrystals cause creep deformation due to this pressure, and voids presenton junction interfaces between the crystals shrink. Thus, the number ofvoids formed between the oxide superconducting crystals is reduced.However, the oxide superconducting wire 1 extends in the longitudinaldirection, and hence force is hardly transmitted in the longitudinaldirection and the wire 1 is hardly compressed in the longitudinaldirection. Consequently, the number of the voids 20 extendingperpendicularly to the longitudinal direction of the oxidesuperconducting wire 1 is hardly reduced even by the heat treatment inthe pressurized atmosphere.

The voids 20 extending perpendicularly to the longitudinal direction ofthe oxide superconducting wire 1, blocking a current in thesuperconducting filament, cause reduction of the critical currentdensity of the oxide superconducting wire 1. When suppressing formationof these voids 20, therefore, the critical current density of the oxidesuperconducting wire 1 can be further improved.

In this regard, the inventors have found that formation of the voidsextending perpendicularly to the longitudinal direction of the oxidesuperconductor can be suppressed before the heat treatment therebyimproving the critical current density of the oxide superconducting wireas a result by setting the draft for the oxide superconducting wire tonot more than 84%, preferably not more than 80% in the primary rolling(step S5) in FIG. 2. The reason for this is now described.

The primary rolling is a step carried out for increasing the density ofthe raw material powder charged into the metal tube. As the draft forthe oxide superconducting wire is increased (the reduction ratio isincreased), the density of the raw material powder charged into themetal tube increases. When the density of the raw material powderincreases, the density of superconducting crystals formed by thesubsequent heat treatments (steps S4 and S5) increases to improve thecritical current density of the oxide superconducting wire.

When the draft for the oxide superconducting wire is increased in theprimary rolling, on the other hand, the following three phenomenaresulting from. the increased reduction ratio may be recognized: First,voids (cracks) are formed in the raw material powder. Second, suchsausaging that the shape of the filament in the oxide superconductingwire is rendered longitudinally heterogeneous is readily caused. Third,such bridging that the superconducting filament comes into contact withanother superconducting filament on a portion where the sectional areathereof locally increases is readily caused due to the sausaging. Allthese phenomena may reduce the critical current density of the oxidesuperconducting wire.

Therefore, the primary rolling must be performed with such a draft thatthe density of the raw material powder increases and no voids are formedin the raw material powder. In conventional primary rolling, an oxidesuperconducting wire has been rolled with a draft of about 86 to 90%.

When the heat treatment is performed in the pressurized atmosphere of atleast 1 MPa and less than 50 MPa, however, an effect of compressing theoxide superconducting wire is obtained also in the heat treatment. Alsowhen the primary rolling is performed with a draft of not more than 84%,therefore, the raw material powder is compressed in the subsequent heattreatment in the pressurized atmosphere and hence the density of thesuperconducting filament of the oxide superconducting wire can beincreased as a result. On the other hand, voids are hardly formed in theraw material powder due to the primary rolling performed with the draftof not more than 84%, whereby formation of voids extendingperpendicularly to the longitudinal direction of the oxidesuperconducting wire can be suppressed. Further, completely no voids areformed in the raw material powder due to the primary rolling performedwith the draft of not more than 80%. The critical current density of theoxide superconducting wire can be improved for the aforementionedreasons.

Referring to FIG. 19, the critical current density of the oxidesuperconducting wire most increases when the primary rolling isperformed with a draft of about 86% if the heat treatment is performedin the atmosphere. If the heat treatment is performed in the pressurizedatmosphere according to the present invention, on the other hand, thecritical current density of the oxide superconducting wire mostincreases when the primary rolling is performed with a draft of about82%. Thus, it is understood that the optimum draft for the primaryrolling for improving the critical current density of the oxidesuperconducting wire shifts to a lower draft side when the heattreatment is performed in the pressurized atmosphere of at least 1 MPaand less than 50 MPa.

In order to confirm the aforementioned effect, the inventors haveprepared oxide superconducting wires in this embodiment under thefollowing conditions, for measuring the critical current densities.

Raw material powder was charged into metal tubes on the basis of thestep of manufacturing an oxide superconducting wire shown in FIG. 2, forperforming wiredrawing. Then, primary rolling was performed forobtaining tapelike superconducting wires. The primary rolling wasperformed with two types of drafts of 82% and 87%. Further, rolls of 100mm in diameter were employed for the primary rolling along withlubricating oil having kinetic viscosity of 10 mm²/s. Then, a first heattreatment was performed on these wires. The first heat treatment wasperformed by setting the partial oxygen pressure to 0.008 MPa, settingthe heat treatment temperature to 830° C. and setting the heat treatmenttime to 30 hours. Then, secondary rolling was performed. The secondaryrolling was performed with a draft of 5 to 30% with rolls of 300 mm indiameter without lubricating oil. Then, a second heat treatment wasperformed. The second heat treatment was performed by setting thepartial oxygen pressure to 0.008 MPa, setting the heat treatmenttemperature to 820° C. and setting the heat treatment time to 50 hours.After the second heat treatment, the critical current densities of theobtained oxide superconducting wires were measured.

Consequently, the critical current density was 30 kA/cm²in the oxidesuperconducting wire worked with the draft of 87% in the primaryrolling. On the other hand, the critical current density was 40 kA/cm²in the oxide superconducting wire worked with the draft of 82%. From theaforementioned results, it is understood possible to suppress formationof voids extending perpendicularly to the longitudinal direction of theoxide superconducting wire before the heat treatment thereby improvingthe critical current density of the oxide superconducting wire bysetting the draft for the oxide superconducting wire to not more than84% in the primary rolling (step S5).

While each of the above embodiments is described with reference to amethod of manufacturing an oxide superconducting wire having a Bi2223phase by hot isostatic pressing, it is also possible to carry out thepresent invention by pressing other than hot isostatic pressing so faras the method performs a heat treatment in a pressurized atmosphere ofat least 1 MPa and less than 50 MPa. Further, the present invention isalso applicable to a method of manufacturing an oxide superconductingwire having another composition such as a yttrium-based compositionother than the bismuth-based composition.

While the step of plating the wire with silver or a silver is carriedout in the second embodiment of the present invention, it is alsopossible to carry out the present invention with a sputtering step, forexample, so far as the step is employed for bonding silver or a silveralloy to the wire. While FIG. 14 and FIGS. 15A to 15D show specificcontrol conditions for the temperature, the pressure, the oxygenconcentration and the partial oxygen pressure, in addition, the presentinvention is not restricted to these conditions but the decompressionspeed for the total pressure in the pressurized atmosphere may becontrolled to not more than 0.05 MPa/min. when the pressure iscontrolled to increase stepwise following temperature rise and thetemperature in the atmosphere is at least 200° C.

When the first to fifth techniques in the second embodiment of thepresent invention are combined with the heat treatment conditions in thefirst embodiment, formation of pinholes can be prevented, or formationof voids and blisters in the wire can be effectively suppressed alsowhen pinholes are formed.

Further, formation of voids and blisters in the wire can be moreeffectively suppressed by properly combining the first to fifthtechniques in the second embodiment of the present invention.

While control is made to compensate for reduction of the pressureresulting from temperature reduction (to add a pressure) in temperaturereduction immediately after the heat treatment in the fifth techniqueaccording to the second embodiment of the present invention, the presentinvention is not restricted to this case but the pressure in theatmosphere may be controlled to continuously increase at least in theheat treatment.

While the exemplary optimum numerical range of the partial oxygenpressure at the heat-up time before the heat treatment and in the heattreatment is shown in the third embodiment of the present invention, thepresent invention is not restricted to the case of controlling thepartial oxygen pressure with this numerical range but the partial oxygenpressure may be controlled to increase following temperature rise in theatmosphere.

While exemplary kinetic viscosity of the lubricating oil in rolling andan exemplary diameter of the rolls employed for rolling are shown in thefifth embodiment, the present invention is not restricted to suchrolling conditions but the draft for the wire in the rolling step may benot more than 84%.

The embodiments disclosed above are considered to be illustrative in allpoints and not restrictive. The scope of the present invention is shownnot by the aforementioned embodiments but by the scope of claim forpatent, and intended to include all corrections and modifications withinthe meaning and range equivalent to the scope of claim for patent.

INDUSTRIAL APPLICABILITY

As hereinabove described, the inventive method of manufacturing an oxidesuperconducting wire is applicable to a method of manufacturing an oxidesuperconducting wire capable of preventing reduction of a criticalcurrent density.

1. A method of manufacturing an oxide superconducting wire, comprising:a step of preparing a wire formed by covering raw material powder of anoxide superconductor with a metal; and a step of heat-treating said wirein a pressurized atmosphere, wherein the total pressure of saidpressurized atmosphere is at least 1 MPa and less than 50 MPa; andcontrolling the total pressure in the atmosphere to increase at a speedof at least 0.05 MPa/min at a heat-un time before the heat treatment insaid heat-treating step.
 2. The method of manufacturing an oxidesuperconducting wire according to claim 1, wherein said heat-treatingstep is carried out by hot isostatic pressing.
 3. The method ofmanufacturing an oxide superconducting wire according to claim 1,wherein said oxide superconductor is a Bi—Pb—Sr—Ca—Cu—O oxidesuperconductor including a Bi2223 phase containing bismuth, lead,strontium, calcium and copper in atomic ratios of (bismuth andlead):strontium:calcium:copper approximately expressed as 2:2:2:3. 4.The method of manufacturing an oxide superconducting wire according toclaim 1, wherein said heat-treating step is carried out in an oxygenatmosphere, with a partial oxygen pressure of at least 0.003 MPa and notmore than 0.02 MPa.
 5. The method of manufacturing an oxidesuperconducting wire according to claim 4, controlling said partialoxygen pressure to increase following temperature rise in saidpressurized atmosphere at a heat-up time before the heat treatment insaid heat-treating step.
 6. The method of manufacturing an oxidesuperconducting wire according to claim 1, controlling the totalpressure in said pressurized atmosphere to be constant in the heattreatment.
 7. The method of manufacturing an oxide superconducting wireaccording to claim 1, wherein said heat-treating step is carried out inan oxygen atmosphere, while controlling the partial oxygen pressure inthe heat treatment to be constant in a fluctuation range within 10%. 8.The method of manufacturing an oxide superconducting wire according toclaim 1, injecting gas to compensate for reduction of the pressureresulting from temperature reduction in the temperature reductionimmediately after the heat treatment.
 9. The method of manufacturing anoxide superconducting wire according to claim 8, wherein said metalcovering said raw material powder includes silver, the ratio of the areaof said metal portion to the area of said oxide superconductor portionin a cross section of said wire after said heat-treating step is 1.5,and a decompression speed in the temperature reduction immediately aftersaid heat treatment is not more than 0.05 MPa/min.
 10. The method ofmanufacturing an oxide superconducting wire according to claim 9,wherein said metal covering said raw material powder includes silver,and the ratio of the area of said metal portion to the area of saidoxide superconductor portion in the cross section of said wire aftersaid heat-treating step is 1.5, for controlling the decompression speedfor the total pressure in said pressurized atmosphere to be not morethan 0.05 MPa/min. when the temperature in said pressurized atmosphereis at least 20° C. in said heat-treating step.
 11. The method ofmanufacturing an oxide superconducting wire according to claim 8,wherein said metal covering said raw material powder includes silver,the ratio of the area of said metal portion to the area of said oxidesuperconductor portion in the cross section of said wire after saidheat-treating step is 3.0, and a decompression speed in the temperaturereduction immediately after said heat treatment is not more than 0.03MPa/min.
 12. The method of manufacturing an oxide superconducting wireaccording to claim 11, wherein said metal covering said raw materialpowder includes silver, and the ratio of the area of said metal portionto the area of said oxide superconductor portion in the cross section ofsaid wire after said heat-treating step is 3.0, for controlling thedecompression speed for the total pressure in said pressurizedatmosphere to be not more than 0.03 MPa/min. when the temperature insaid pressurized atmosphere is at least 20° C. in said heat-treatingstep.
 13. The method of manufacturing an oxide superconducting wireaccording to claim 1, controlling the decompression speed for the totalpressure in said pressurized atmosphere to be not more than 0.05MPa/min. when the total pressure of said pressurized atmosphere is atleast 1 MPa in said heat-treating step.
 14. The method of manufacturingan oxide superconducting wire according to claim 1, further comprising astep of rolling said wire with a roll after said step of preparing saidwire and before said heat-treating step, wherein the integumentarythickness of said wire after said rolling step is at least 20 μm. 15.The method of manufacturing an oxide superconducting wire according toclaim 1, further comprising a step of bonding silver or a silver alloyto the surface of said wire after said step of preparing said wire andbefore said heat-treating step.
 16. The method of manufacturing an oxidesuperconducting wire according to claim 1, further comprising a step ofrolling said wire with a roll after said step of preparing said wire andbefore said heat-treating step, wherein the maximum height Ry ofirregularities is not more than 320 μm as to the surface roughness of aportion of said roll coming into contact with said wire.
 17. The methodof manufacturing an oxide superconducting wire according to claim 1,controlling the pressure to increase stepwise following temperature risein the atmosphere at a heat-up time before the heat treatment in saidheat-treating step.
 18. The method of manufacturing an oxidesuperconducting wire according to claim 1, controlling the totalpressure in said atmosphere to continuously increase in the heattreatment in said heat-treating step.
 19. The method of manufacturing anoxide superconducting wire according to claim 1, further comprising astep of rolling said wire after said step of preparing said wire andbefore said heat-treating step, wherein the draft of said wire in saidrolling step is not more than 84%.
 20. The method of manufacturing anoxide superconducting wire according to claim 19, wherein the draft ofsaid wire in said rolling step is not more than 80%.
 21. The method ofmanufacturing an oxide superconducting wire according to claim 1,wherein a plurality of heat treatments are performed on said wire, andat least one heat treatment among said plurality of heat treatments iscarried out in a pressurized atmosphere having a total pressure of atleast 1 MPa and less than 50 MPa.